This invention relates to focusing systems for photographic cameras and, more particularly, it concerns a focusing system and method by which the variable depth of focus of a camera lens is accounted for automatically in the focus setting of the lens.
Automatic focusing systems for photographic cameras are now well known as evidenced by such commercially available cameras as those marketed by Polaroid Corporation of Cambridge, Mass. under such names as "POLAROID SX-70 SONAR ONE-STEP" and PRONTO ONE-STEP." In such cameras, camera/subject distance is determined by the time required for an acoustical pulse to travel to and from a subject to be photographed. Once the distance or range has been determined, the objective lens of the camera is driven by an electric motor to a focus position corresponding to the acoustically determined range and the film exposure cycle of the camera is carried out. The entire operation of range finding, focusing and exposure occurs in a fraction of a second so that the time required for focusing does not add noticeably to the time normally associated with camera operation for film exposure.
The range finding and lens adjusting functions of such systems are accomplished by electronic circuitry including a focus control pulse counter, a clock oscillator which, when activated, transmits in accordance with subject range, a series of pulses to the counter, and a lens-associated encoder wheel/detector which generates and transmits to the same counter a number of pulses representative of lens focusing movement from a known initial position to the proper focus setting. As camera/subject distance decreases, the number of pulses from the clock oscillator will decrease. Since the lens movement is from the infinity setting to the nearest focus distance setting, the number of pulses resulting from lens focusing movement will increase inversely in relation to camera/subject distance. As a result of this organization of components, lens focusing movement is terminated in response to the focus control counter accumulating a predetermined number of counted pulses.
Also, in cameras of this type manufactured by Polaroid Corporation, a light integrating exposure control system is used to determine automatically the quantity of light permitted to pass to the film during exposure. While like conventional cameras, proper exposure for a given film speed is determined by a combination of aperture size and shutter speed, these two parameters are combined or integrated in a single pair of shutter blades positioned directly behind the objective lens of the camera. When actuated for exposure, these blades move from a closed position through a progressively enlarged aperture setting until they stop and return to a closed position. Auxiliary openings in the same set of blades sweep a light detecting photocell associated with a light integrating network which determines the aperture size to which the shutter blades will move before returning to an exposure terminating or closed position. This type of exposure control is exemplified by the disclosures of U.S. Pat. Nos. 3,641,889, issued Feb. 15, 1972 to Viato K. Eloranta and 3,975,744, issued Aug. 17, 1976, to Bruce K. Johnson, et al.
As in all variable aperture or multi-stop lenses, the distance along the lens axis within which objects will be focused on the camera film plane for a given focus setting of the lens, or depth of focus, will vary with the aperture stop or diameter of the lens opening through which light is passed for film exposure. Specifically, the depth of focus varies inversely with aperture size. In cameras equipped with a light integrating exposure control system of the type mentioned, therefore, depth of focus will vary directly and automatically with the brightness of the scene or object to be photographed.
In acoustical lens focusing systems of the type mentioned, the camera/subject distance is the distance between the camera and the nearest sound reflecting subject portion or object located close to the optical axis of the camera lens. The lens will be focused, therefore, at a position such that the nearest subject portion will lie midway between the near and far limits of the in-focus distance or field provided by the lens depth of focus. While this condition is perhaps representative of the focusing position to which the lens would be set manually by most amateur photographers, in many cases it would be desirable for the medial plane of the in-focus field to be positioned behind the nearest subject so that the near limits of the depth of focus distance falls closely in front of the nearest object while the far limit of the in-focus distance is extended rearwardly. This situation is illustrated by three objects constituting a total subject to be photographed and positioned, for example, at seven, nine and twelve feet from the camera and illuminated by ambient light calling for a lens aperture at which the depth of focus distance is approximately five feet. If the camera lens is focused for approximately seven feet as a result of an acoustical ranger finder distance determination, the first two objects will be in focus whereas the object located twelve feet from the camera will be out of focus. If the focus setting of the lens is increased to nine and one-half feet, all three objects would be in focus as a result of the depth of focus under the ambient light conditions indicated.